MARK COLVIN: David Hicks spent six years under lock and key, mostly in Guantanamo Bay but now the former Chief Prosecutor at the US military prison says he should never have been charged.

Colonel Morris Davis has now been called as a defence witness at a pre-trial hearing for one of the detainees.

It's the latest twist in the long-running efforts by the Bush administration to get trials up and running at Guantanamo.

Colonel Davis was originally a staunch supporter of the military commissions set up to try Guantanamo prisoners but he resigned last year shortly after Hicks was convicted, claiming political interference.

Today, he appeared on the witness stand at Guantanamo Bay at a hearing for the next prisoner due to stand trial, Salim Ahmed Hamdan, who's alleged to have been Osama bin Laden's driver.

LEIGH SALES: Colonel Morris Davis used to be a huge believer in the military commissions, as their Chief Prosecutor.

But once he'd worked under the system for a while, he changed his mind, like many other who've gone before him.

Colonel Davis claims there was political interference in the cases being selected for trial.

He told a hearing at Guantanamo today that Bush administration appointees lobbied for charges in particular cases, because they'd bolster public support for Guantanamo.

Colonel Davis also disapproved of the abusive techniques used to elicit evidence from prisoners.

Lawyers for detainee Salim Ahmed Hamdan called Colonel Davis to appear at his hearing today, arguing that the former Chief Prosecutor's testimony is grounds for the case against Hamdan to be thrown out.

Australian David Hicks didn't contest the charges against him and since he was convicted in 2007, not one other detainee has faced trial.

Some have now been held for more than six years without a day in court.

Hamdan is supposed to be the first trial, in May.

But there's much discussion in national security circles in Washington about the inevitability of Guantanamo Bay's closure, given the three remaining Presidential candidates, Hillary Clinton, Barack Obama and John McCain, all say the facility should be shut down.

The question then becomes what to do with the prisoners who can't be released, including high level terrorist suspects such as Khalid Sheikh Mohammed, the alleged 9-11 mastermind, and Hambali, the alleged architect of the Bali bombings.

Those men can't be brought to trial in regular courts because any charges against them would be thrown out because of the irregular and abusive ways they've been detained and interrogated.

Two academics who've been staunch critics of the military commissions have floated a new idea of a National Security Court in the US, using sitting Federal Court judges.

The US Government could produce classified information to argue why certain prisoners should be detained indefinitely.

The defendants would have lawyers, although they'd have fewer rights than in regular criminal cases.

Civil libertarians oppose the proposal, but it may appeal to the Bush administration or to a future President McCain, Obama or Clinton, given the option of simply letting all the Guantanamo inmates free is politically unpalatable and arguably strategically unwise.